I was never much of a fan of the mystery genre, but many gay kids liked the gentle, pre-Hardy Boys exploits of The Famous Five or their American counterparts, the Bobbsey Twins and the Boxcar Children.
Laura Lee Hope’s Bobbsey Twins series lasted through 72 installments from 1904 to 1979. Originally the two sets of twin siblings aged normally, but when the series was revised and extensively rewritten during the 1960s, Bert and Nan remained twelve (but behaved as adolescents), and Freddie and Flossie remained six (they all seemed to behave somewhat older than their "real" ages, or maybe that is just a reflection of the extra freedom kids had in earlier generations). In the 1960s they also began to have more dramatic adventures in realistic locales, though the titles were still aimed at a youngish market: The Secret of Candy Castle, The Doodlebug Mystery, The Flying Clown.
Gay boys found most resonance in Bert, who was in his last days of childhood, still happy to play with his sister and younger siblings but obviously longing for emotional connections outside the group. In fact, an ongoing theme of the books is the conflict between the comfort and safety of family and the need to “leave the nest” and find one’s own way in the world. But girls play no part in any of the stories; instead, in nearly every book, in the midst of piecing out clues and solving mysteries, Bert goes off on his own with a boy.
The Boxcar Children were another group of siblings, Henry (14), Jessie (13), Violet (10), and Benny (6), orphans who moved into an abandoned boxcar in the 1924 novel by Gertrude Chandle Warner. Then, in the late 1940s, Warner realized that the four would make ideal child-sleuths. She had them adopted by their wealthy grandfather, Mr. Alden, who traveled around the country to keep track of his various business investments, thus providing lots of exotic locales for sleuthing. Eighteen new installments appeared between 1949 and 1976, sending the kids to haunted houses, bedeveled ranches, mountain cabins, and seaside resorts. The children age through the adventures, and by #19, Benny Uncovers a Mystery, Henry is in college.
Like Bert, Henry is trying to establish his independence while still remaining part of the family, but, unlike adolescent boys in children's media today, he is never portrayed as girl-crazy. Instead, when his life outside the family appears in the novels, he is usually seen in the company of a boy (the girl on this cover is his sister).